The invention relates to a hydraulic system for machines provided with tools, particularly for wheel loaders, fork lifts or the like, having a load-damping system comprising at least one hydraulic accumulator connected to the hydraulic lines responsible for lifting and lowering the tool and that extend between the lifting cylinder and a control valve.
Construction machines having pneumatic tires must often travel a long distance when they are to be used at a construction site. They can be driven between construction sites and to their locations of use, because they fulfill the conditions of admission for participation in public traffic, even with trailers from time to time.
The driving speeds that can be attained during use contribute significantly to the transport capability and thus the economical aspect of the machine. However, even for tools that must be transported frequently between construction sites, or must use lengthy routes to reach these sites, the time required to do this is a significant factor in the cost calculation of the contractor.
The driving speed of this type of machine is not limited by the engine capability--with the exception of driving on steep gradients--but by the vibrations the vehicle experiences due to unevenness of the ground. The driver is thus obligated to select a speed considerably below the speed that could be attained. The primary cause of the "bumping" of the machine is the lack of a spring system. Up to now, spring systems have only been constructed in construction machines for special purposes, for example in military applications with the requirement of speeds up to over 60 km/h. The reasons these types of construction machines are built without spring systems are, on the one hand, that a spring system, because of its yielding under lifting and tensile forces, would be disadvantageous during loading. On the other hand, installing a spring system represents a relatively high construction expenditure that would by nature have to result in considerable additional costs.
From DE-C 3,909,205, a hydraulic system is known for construction machines, particularly wheel loaders, tractors and the like, that include a tool, particularly a loading shovel, that is operated by a hydraulic cylinder, wherein a main line is provided for operating the hydraulic cylinder that leads from a pressure source to the hydraulic cylinders via a control valve, from which line a connecting line that leads to at least one hydraulic accumulator branches off, and in which a switchable check valve is disposed. A feed line is provided that bridges the check valve and connects the main line to the hydraulic accumulator, and a pressure-reducing valve is disposed in the feed line. The pressure-reducing valve is set to the carrying pressure of the hydraulic cylinder, and is preferably configured as a pressure-limiting valve or as a pressure cut-off valve. The switchable check valve is configured as a magnet valve that is controlled as a function of the driving speed or the tilting angle of the tool, wherein during driving speed-dependent control of the magnet valve, the switching point is set such that it cannot be exceeded until second gear is reached.
Because only one predeterminable carrying pressure (e.g. 120 bar) can be set in the use of pressure-reducing valves, which cannot be viewed as being a realistic value in every working state, the load-damping system used here is viewed as inadequate for all operating states of the machine. Moreover, the gear- or driving speed-dependent switching of the pressure-reducing valve likewise cannot optimally manage the pitching vibrations established in the operating state.